For cellular radio systems numerous solutions have over the years been suggested for power control in enhanced uplink (EUL). Recently, Signal to Interference Ratio (SIR) based methods have been questioned with the argument that power control aiming for a constant received power gives a calmer and more stable system behavior. This has been of particular interest for Time Division (TD) scheduling because a future power control solution is likely to need to work in combination with a TD scheduler.
Conventional power control schemes are often SIR based, which dates back to Wideband Code Division Multiple Access (WCDMA) release 99 where no user was a dominating source of interference. Since a large number of users contributed to the total interference, the change in SIR of an individual user did not change the interference enough for it to have any significant effect on the SIR of the other users. The inner loop power control typically aims at retaining a target SIR, which in turn is set by the much slower outer loop power controller. However, a SIR based power control scheme has the advantage that it guarantees a certain signal quality for each user. Inner loop power control can be defined as the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, striving to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target.
One alternative to SIR based power control is to try to retain constant received signal code power (RSCP) on Dedicated Physical Control Channel (DPCCH). Again this is typically obtained by comparing a target, in this case the target RSCP, to the measured equivalent, and then request that a User Equipment (UE) adjusts its transmitted power up or down. The main advantage of this method is that as the Enhanced Uplink (EUL) scheduler gives grants it effectively determines the amount of received power (at the radio base station Node B) each UE is allowed to generate. By using RSCP based power control one can be sure that the UE will not violate the power budget and power peaks will not be induced by the power controller.
A problem with some existing SIR based power control methods is that all users are linked to each other and that this is not taken into account in the inner loop power control. The users may then repeatedly trigger each other to increase the power levels, leading to power peaks and long settling times. In addition, there is typically more than one set of powers to achieve a specific set of SIRs for the users. The consequence of a power peak can then be that all users end up transmitting with higher power than necessary, generating extra interference inside and outside the cell where the users are located.
For power based control methods, a problem is that there is no way to guarantee the signal quality. Where SIR based power control increases the power to compensate for deteriorating signal quality as the interference increases, the power based control compensates for fading but has no way to compensate for changing interference. The result will then be an increased number of block errors until the interference subsides or the RSCP target is adjusted via the much slower outer loop control.
Hence there exist a need for new methods and devices providing improved performance for control in the inner loop of a cellular radio system.